This functionality is available for the Pie Chart via the plotOptions size parameter, however after inspecting the Highcharts API the size isn't available for the Gauge.
size API entry for Pie Chart
Considering the likeness of the two charts (being circular), I was hoping there would be a similar option but the gauge always fills some subset of the container div that I am unable to control. Has anyone else run into this issue or found a solution?
By setting the spacing to zero, you can fill the entire space:
http://jsfiddle.net/mkremer90/3sngK/1/
Fiddle with the spacing
parameters to change how much the chart fills. As for a percentage based like size
option I can't find one either.